Topic 3 - Amount of substance

Question 1

Define percentage atom economy

Answer 1

molecular mass of desired product/ sum of molecular masses of all reactants x 100

Question 2

Define percentage yield

Answer 2

Actual mass produced / theoretical mass x 100

Question 3

Define empirical formula

Answer 3

Simplest whole number ration of atoms of each element in a compound

Question 4

Define mole fraction

Answer 4

Number of moles of gas A / Total number of moles of gas

Question 5

Define partial pressure

Answer 5

Mole fraction x total pressure

Question 6

How to make a standard solution?

Answer 6

Work out how many moles you need using the formula n=cv

Then work out how many grams of solid you need using mass = n x mr

Weigh this mass of solid using a weighing boat and by weigh by difference

Add distilled water and stir until all solid has dissolved in the beaker

Using a funnel tip the solution into a volumetric flask.
Rinse beaker, glass rod and funnel with distilled water and tip into volumetric flask.

Top up VF with distilled water to the correct volume - bottom of meniscus on the line - drop by drop

Stopper the flask and invert to ensure an even concentration throughout

Question 7

Titration method

Answer 7

Using a volumetric pipette measure out a set volume of the solution you want to know the concentration of

Add to a conical flask and add a few drops of an appropriate indicator:
- methyl orange - red in acid and yellow in alkali
- Phenolphthalein - colourless in acid and pink in alkali

Then fill the burette with the standard solution (known conc and volume) - use a funnel

First do a rough titration to get a rough idea of the end point.

Then do accurate titrations - let the burette run until within 2cm3 of the end point then do drop by drop whilst swirling.
When there is a colour change and work out the amount of standard solution required for neutralisation

Repeat titrations until concordant results and calculate a mean titre